1. Field of the Invention
The present invention relates generally to a gear and, more particularly, to a gear for eliminating backlash, in which a second gear is rotatably connected to the front of a first gear, cylinders for rotating the second gear are provided on the first gear, so that clearance between the first and second gears rotating identically with a rotating body rotating forwards and backwards and another gear engaging with the first and second gears is eliminated, thus eliminating backlash, therefore allowing rotating force to be reliably and continuously transmitted by the first and second gears which are rotating forwards and backwards.
2. Description of the Related Art
Generally, when a pair of gears mesh with each other, theoretically, respective gear teeth engage with corresponding tooth spaces. However, actually, clearance occurs between the gear teeth and the tooth spaces because of errors in the manufacturing of gears.
Since power transmission is not smooth because of various factors including variation in radial distances of the gears resulting from manufacturing error, the deformation of the tooth profile or the abrasion of a gear shaft due to a load, and assembly tolerance, some clearance occurs in the back of gear teeth when they engage with each other. That is, backlash occurs.
In other words, backlash is defined as the clearance between the gear teeth in a circumferential direction when the gears engage with each other. When there is no clearance, the gear teeth having the same dimensions cannot engage with each other, so that production tolerance is always present.
Backlash occurs when a pair of gears which engage with each other and rotate in one direction rotate in the opposite direction. The backlash occurs between the gears rotating forwards and backwards, thus undesirably increasing stress and abrasion.
In an optical pickup apparatus using a recording medium rotating forwards and backwards such as a CD or DVD, power transmission for driving light pickup operation is conducted mainly by gears. If backlash occurs between the gears for transmitting power in the optical pickup apparatus, the backlash negatively affects the operation and reliability of the optical pickup apparatus.
That is, when backlash is present between the power transmission gears of the optical pickup apparatus, an error is made in power transmission. Further, play occurs in the pickup operation due to external shocks, so that the pickup operation may deviate from a track of a CD or DVD data surface, thus hindering the data from accurately playing.
In order to solve the problem, various methods have been developed. Among them, a gear for eliminating backlash was developed as shown in FIG. 1 and has been used.
FIG. 1 is a view illustrating a conventional double gear for eliminating backlash.
As shown in the drawing, two gears having the same dimensions are coaxially connected to each other. A support member 1a is formed at a predetermined position in the upper surface of the lower gear 1, and an elongated hole 2a is formed at a predetermined position in the upper gear 2. Further, a protrusion 2b is provided on one wall in the hole 2aso that a compression spring 3 is installed between the support member 1a of the lower gear 1 and the protrusion 2b. 
Further, locking projections 1b are provided on a plurality of places on the upper surface of the lower gear 1, and guide grooves 2c are formed on a surface of the upper gear 2 to correspond to portions on which the locking projections 1b are formed. The locking projections 1b are inserted into the guide grooves 2c. 
In such a conventional double gear for eliminating backlash, the upper gear 2 is pushed in one direction by the elastic force of the spring 3 installed in the elongated hole 2a of the upper gear 2. Such a movement of the upper gear 2 is limited when each locking projection 1b is locked to one wall of the corresponding guide groove 2c. 
Thus, the elastic force of the spring 3 is always applied to the upper gear 2. The teeth of the upper and lower gears 2 and 1 are staggered. Thus, both sides of a tooth space of another gear engaging with the teeth of the upper and lower gears 2 and 1 are in close contact with the teeth because of the elastic force of the spring 3 which acts on the upper gear 2, so that backlash is minimized.
However, the conventional double gear for eliminating backlash is problematic in that the elastic force of the spring 3 pushing the upper gear 2 in one direction is reduced when the double gear has been used for a lengthy period of time, so that force for continuously pushing the upper gear 2 becomes weak.
This means that the life-span of the conventional double gear for eliminating backlash is short. Therefore, the double gear must be separated from a product so as to replace the spring 3 with a new one, or the product must be replaced with a new one, thus inconveniencing a consumer and deteriorating reliability.